Legal nullity is a phrase used to refer to any entity which might theoretically be of some legal significance, but in fact possesses no identity or distinct structure of its own.
Examples of this are counties which are wholly subsumed by the municipal government within their boundaries.
Some entities which fit this description are Philadelphia County, a legal nullity because it is entirely coterminous with the city of Philadelphia, Pennsylvania and New York County, which is similarly coterminous with the Borough of Manhattan, in New York City (as are each of the Five Boroughs of New York City).
Legal nullity is also a court decision made without jurisdiction or authority. Such a decision does not have to be vacated or appealed, it is void from inception.